I see this a lot, but end up ignoring it and letting him do his own thing in toplane. I'm just wondering though, if he proxy farms, won't he die a lot? Isn't this going to benefit the enemy team?
The point of proxy farming is to provide a point of pressure on the map and to make it hard for your opponent to properly farm minions.
In the case of Singed in particular, he can clear entire waves pretty easily even very early in the game (level 1-3) compared to most champions, causing his lane opponents to have to try and fight the turret for last hits on minions (which is difficult early on when you have little to no bonus ad/ap).
This forces the enemy team to either respond and allow singed's team easy roaming or invading power, or to ignore it and singed will constantly be getting damage on the top lane turret by crashing minions into it. Either of these provide advantages to singed's team, and in the case of a good player, singed usually can escape early ganks using a combination of ghost, a slow/root, and a displacement ability which only leads to the enemy wasting further time, experience, and gold and gives your team a pressure advantage on the other side of the map.